The present invention relates to a vehicle having hydrostatically driven ground wheels and more particularly relates to controls for a dual-path hydrostatic drive system.
Self-propelled windrowers are typically driven through a dual-path hydrostatic drive system. That is to say, each of a pair of drive wheels is independently driven by a hydrostatic pump and motor set. The displacement and direction of fluid delivery of the pumps to the motors may be varied so as to effect changes in the speed and direction of output of the motors by adjusting the position of pump control arms in opposite directions from a "neutral" position. Steering is accomplished by causing one motor to be driven at a slower speed than the other by moving the control arms in opposite directions so that one pump displaces less fluid than the other, while speed/direction changes are accomplished by changing the speed/direction of both motors in unison by moving the pump control arms in unison to opposite sides of their respective "neutral" positions. On self-propelled windrowers, the operator interface with the pump and motor sets for accomplishing steering is typically by way of a steering wheel, while the interface for accomplishing speed/direction changes is typically a lever.
It is desirable to keep the steering and speed/direction control separate from each other so each may be done without affecting the other. One known way of achieving this is by providing a more-or-less horizontal steering control shaft having a pair of threaded areas of opposite hand onto which threaded collars respectively of a pair of pump control levers are received. The steering control shaft is coupled for being rotated by a steering wheel so that the pump control arms are pivoted in opposite directions for effecting steering in a desired direction corresponding to the direction of rotation of the steering wheel. Speed/direction changes are made by moving the steering control shaft fore-and-aft, thus effecting pivoting of the pump control arms in unison in the same direction. A disadvantage of this type of control mechanism is that the steering rate (number of turns lock-to-lock) of the steering wheel is limited by the pitch of the threads on the steering control shaft and the practical length limits of the pump control arms. Other disadvantages of this known control mechanism include the maintenance required due to using threaded joints in a dirty environment and of the mechanism not being easily adaptable for use with centering devices for returning the steering wheel to a centered position for effecting straight ahead travel when released.
Another known control system, which is provided with a mechanism for controlling steering which overcomes some of the disadvantages the threaded shaft type steering control mechanism, includes a pivotally mounted control plate coupled in a symmetrical fashion to the pump control arms with pivoting of the plate causing opposite movement of the arms. so as to slow down one drive wheel while increasing the speed of the other. Speed and direction changes in these systems are effected by a manually-operable control lever shiftable in opposite directions from a "neutral" position and coupled for causing the plate to be shifted substantially linearly in opposite directions so as to cause the pump control arms to move in concert in corresponding opposite directions. In one such control system, pivoting of the plate is accomplished by a length of roller chain extending from a relatively small sprocket that is coupled to a steering wheel controlled steering shaft, with free ends of the chain being coupled to opposite locations of the pivotally mounted control plate. However, this control system too has a serious shortcoming. Specifically, ground speed adjustments require a change in the length of the roller chain used to transmit motion of the steering shaft to the pivot plate. Although, this length change is absorbed through a spring located in the coupling joining one end of the chain with the pivot plate, a steering correction must be made for each ground speed adjustment. Furthermore, due to its many pivot joints, the chain introduces opportunities for looseness into the system which adversely affects the steering characteristics.
Still another known control system incorporates a rack and pinion arrangement coupled to the steering shaft for controlling steering. The pump control arms are moved through a series of linkages. Speed and direction is likewise controlled through a series of linkages. The main disadvantage of this type of system is that the number of joints or pivots and linkages used introduces opportunities for looseness in the system resulting in a complication of adjustments.